The present invention relates generally to a vehicular transmission which comprises a transmission capable of varying its speed change ratio and of transmitting the driving force output from the engine to the drive wheels of a vehicle.
Such vehicular transmissions have been used for mechanical power transmission in vehicles. As speed change mechanisms incorporated in these transmissions, not only gear transmissions with a plurality of speed ratios but also continuously variable transmissions with a belt, etc. have been used for speed change control. Recently, a hybrid transmission which uses an electrical motor in addition to an engine has been introduced for fuel efficiency. The applicant of the present invention is also developing a hybrid transmission. This hybrid transmission comprises a continuously variable transmission with a metal V-belt which is disposed around the common output shaft of an engine a and an electrical motor arranged in series, and the output shaft of the continuously variable transmission is provided with a forward/reverse switching mechanism and a starting clutch (main clutch).
One purpose for developing this transmission is to improve fuel efficiency. Therefore, there is a consideration that the operation of the engine be controlled such that it stops when the vehicle has stopped (referred to as xe2x80x9cidling eliminationxe2x80x9d). As such idling elimination control, there is a method to stop the engine when the engine comes into idling after the vehicle has stopped completely. However, when the vehicle is decelerated by releasing the accelerator pedal that has been depressed, an engine brake is effected. It is known that, during this deceleration, the fuel injection to the engine is terminated (or the supply of fuel is terminated). In this situation, there is a more desirable way to eliminate engine idling. When the vehicle is decelerated to a halt, the fuel injection is terminated, so this condition should be maintained to stop the engine. This method seems more favorable for improving the fuel efficiency.
When the engine is stopped to avoid engine idling, the transmission is set at a LOW ratio, and the supply of electricity to the electromagnetic valves (solenoid valves) which control the speed change operation may be stopped (or the electrical currents supplied are reduced almost to zero) because there is no need to perform any speed change. While the engine is not operated, preferably, the supply of electricity to such electromagnetic valves be terminated, and the charge in the battery be conserved for the time being. In this way, the charge in the battery can be used more advantageously to power the electrical motor in an effective way. Generally, the electromagnetic valves are disposed between a hydraulic pump which is driven by the engine and hydraulically operated actuators which systematically set the speed change ratio of the transmission. The spool of each of these valves is disposed in a fine balance of biasing forces which are generated by a resilient member such as spring, by the electromagnetism created by a current through the solenoid of the valve and by a back pressure fed back from the control pressure that controls the actuation of an actuator, and they are systematically controlled to change the control pressures which effect the speed change operation of the transmission.
If the operation of the engine is being terminated to avoid idling, and when the rotational speed of the engine decreases below the idling rotational speed of the engine, the output of the hydraulic pump, which is driven by the engine, attenuates. As the pressure of the hydraulic circuit of the transmission decreases, the back pressure acting on each electromagnetic valve also decreases. In this situation, once the pressure of the hydraulic circuit has begun decreasing, even though the electrical current necessary for maintaining the speed change ratio of the transmission at a LOW ratio is provided continuously, the spool of each electromagnetic valve starts to shift in the direction of the force generated the resilient member because the decreasing back pressure creates a change in the above described balance. As the spool comes close to the wall of the housing of the respective valve, it happens to cause a chattering, which is a displeasing vibration and a noise.
It is an object of the present invention to provide a control system for a vehicular transmission which system is capable of preventing electromagnetic valves from chattering when the pressure of the hydraulic circuit of the transmission decreases as the operation of the engine is terminated to avoid engine idling.
In order to achieve this objective, the present invention provides a first embodiment of control system for a mechanical power transmission which is used for driving a vehicle. This control system comprises an engine, a speed change mechanism (for example, the continuously variable transmission CVT of the embodiment described in the following section), a hydraulic pump and an electromagnetic valve (for example, the drive-pulley electromagnetic valve 45 and the driven-pulley electromagnetic valve 46 of the following embodiment). The speed change mechanism transmits the driving force of the engine with speed change, and the hydraulic pump is driven by the engine to deliver hydraulic oil. The electromagnetic valve is disposed between a first oil passage (for example, the oil passage 103a and the oil passage 103b of the following embodiment) and a second oil passage (for example, the oil passage 107 and the oil passage 108 of the following embodiment). In this arrangement, the first oil passage leads to the pump while the second oil passage leads to a speed change actuator (for example, the variable width drive pulley 11 and the variable width driven pulley 16 of the following embodiment) of the speed change mechanism. The electromagnetic valve adjusts the pressure of the oil to control the speed change actuator in a balance of first, second and third biasing forces and delivers this control pressure into the second oil passage. Here, the first biasing force is generated by a resilient member, the second biasing force is generated electromagnetically, and the third biasing force is generated by the back pressure from the second oil passage. In this arrangement, the system according to the present invention controls the vehicular transmission in the following way. After the rotational speed of the engine has become below a reference rotational speed which is lower than an idling rotational speed and after the pressure of the first oil passage begins to decrease, for a predetermined time, the current to the electromagnetic valve is adjusted and supplied to generate the second biasing force in a magnitude that can supplement a decrease in the third biasing force, which decrease is caused from a decrease in the pressure of the first oil passage. Then, after the predetermined time has elapsed, the current supplied to the electromagnetic valve is set almost to zero. The above mentioned predetermined time corresponds, for example, to a time which takes, after the pressure of the first oil passage begins to decrease, for the spool of the electromagnetic valve to be pushed and shifted toward the wall of the housing of the valve by the resilient member as the third biasing force decreases, so as to result in a chattering with the wall.
In this first embodiment of control system according to the present invention, if the rotational speed of the engine decreases below the reference rotational speed, which is lower than the idling rotational speed of the engine, for example, in an engine idling elimination control, and when the pressure of the first oil passage begins to decrease, for the predetermined time, the current to the electromagnetic valve is adjusted and supplied to generate the second biasing force in a magnitude that can supplement the third biasing force which decreases in correspondence with the decreasing pressure of the first oil passage. Therefore, it is not possible for the spool of the electromagnetic valve to shift in correspondence to the decrease in the pressure of the first oil passage, so no chattering of the spool with the wall of the housing will occur. Thus, the control system according to the present invention effectively prevents the electromagnetic valve from causing any displeasing vibration and noise. In addition, after the elapse of the predetermined time, the current to the electromagnetic valve is set almost to zero. By terminating the cause of chattering in this way, a further prevention is made against vibrations and noises. As described above, even though the engine rotational speed becomes below the reference rotational speed, the electromagnetic valve is supplied with a current which is sufficient to maintain the speed change ratio of the transmission at a LOW ratio until the pressure of the first oil passage begins to decrease. In this way, even in a case where the transmission comprises a belt type continuously variable transmission, a predetermined pressure is secured for controlling pulleys which are incorporated in the transmission, thereby preventing the belt from slipping.
A second embodiment of control system according to the present invention comprises an engine, a speed change mechanism, a hydraulic pump and an electromagnetic valve. The speed change mechanism transmits the driving force of the engine with speed change, and the hydraulic pump is driven by the engine to deliver hydraulic oil. The electromagnetic valve is disposed between a first oil passage and a second oil passage, and the first oil passage leads to the pump while the second oil passage leads to a speed change actuator of the speed change mechanism. The electromagnetic valve adjusts the pressure of the oil to control the speed change actuator, in a balance of first, second and third biasing forces, and delivers this control pressure into the second oil passage. Here, the first biasing force is generated by a resilient member, the second biasing force is generated electromagnetically, and the third biasing force is generated by the back pressure from the second oil passage. In this arrangement, the system according to the present invention controls the vehicular transmission in the following way. After the rotational speed of the engine has become below a reference rotational speed which is lower than an idling rotational speed and after the pressure of the first oil passage begins to decrease, the current supplied to the electromagnetic valve is set almost to zero.
In this second embodiment of control system according to the present invention, if the rotational speed of the engine decreases below the reference rotational speed, which is lower than the idling rotational speed of the engine, for example, in an engine idling elimination control, and when the pressure of the first oil passage begins to decrease, the current to the electromagnetic valve is adjusted almost to zero. By terminating the cause of chattering in this way, a prevention is made against vibrations and noises. As in the above described first control system, also in this control system, even though the engine rotational speed becomes below the reference rotational speed, the electromagnetic valve is supplied with a current sufficient to maintain the speed change ratio of the transmission at a LOW ratio until the pressure of the first oil passage begins to decrease. In this way, even if the transmission comprises a belt type continuously variable transmission, a predetermined pressure is secured for controlling pulleys which are incorporated in the transmission, thereby preventing the belt from slipping.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.